This is how I do ballistic hamstring stretches nowadays, it was inspired by the Phil Lancaster post and a little experimentation:

Sit down, pull your right knee to the chest and lay your torso onto it. Your knee will be at around 90 degree or less.

Let the left leg take care of itself, it will likely exorotate to the side, it is okay.

Hold your right foot (if you can't reach your foot in this position use a towel) with boths hands and do the following:

Using your calve pull yourself into a stretch. These are very slight gently and rhythmic pulling or pulsing actions. Do this for 20--25 reps with 2--3 reps/sec.

Relax

Straighten your right leg ~10cm

Repeat the gently pulses.

Repeat this two or three times, always straightening the leg a couple of additional cm

After the last set place your hands on the foot (use a ball if you cannot reach it) and try to push it away while simultanously pulling your foot with the shin muscles in direction or your body. Do the latter with a similar frequency as the first pulses. So 20--25 gently pulses while you push against your foot. As the direction of the pulses is reversed, you are no longer pulling yourself into a gentle stretch but out of it, but you will still gain ROM during this last cycle

Now switch legs and repeat the whole procedure with the left leg.

When you are done repeat the whole process with both legs at the same time.

This works best when you place a pillow under the foot of the leg your are 'stretching' so it's a little elevated. If you like you can bend over both legs before switching to the left side, the difference in perceived tension will likely be dramatic.